Peripherally spaced tool holders of an excavating head of the type here considered, e.g. as illustrated in U.S. Pat. No. 3,747,982, are provided with sockets in which the shanks of respective tools or picks are removably received; the holders may be welded onto the body of the rotary head. The shanks may be rounded or flat, depending on whether or not the picks should be free to rotate about their own axes. Such rotatability is desirable for a substantially uniform all-around wear of their cutting tips; the latter usually consist of hard metal and have substantially frustoconical working edges. When that wear becomes excessive, the picks can be extracted from their sockets and replaced by fresh ones.
The recurrent encounter of a given pick with the mineral matter to be fragmented, e.g. at a mine face, subjects its shank to considerable stresses which also act upon the surrounding socket wall. These stresses may lead to deformations preventing further rotation of the shank in its socket so that the cutting tip of the pick begins to wear unsymmetrically and deteriorates more rapidly. Such a blocking of rotation can also be the result of comminuted matter penetrating into and becoming wedged in the narrow annular gap which separates the shank from the socket wall.
As known in the art, the tips of the picks as well as the mine face or other mineral formation attacked thereby ought to be cooled by a water spray during excavation. Such a spray also serves for the preceipitation of coal dust or the like developing during fragmentation and facilitates the removal of detritus from the site. For economic reasons, and to simplify the recovery of valuable minerals such as coal by avoiding the need for a separate drying step, it is desirable to hold the water supply within moderate limits. Thus, rotary heads with picks projecting parallel to the axis of rotation have been provided in the past with valves which block the flow as long as the picks are not in contact with a mine face. The valve responds to pressure exerted in the direction of that axis and this unblocks the flow intermittently as the tool is progressively advanced.
With tool heads having generally tangentially oriented picks, to which my invention is applicable, such a control cannot be used since the picks are not shiftable in the direction of advance. Moreover, each individual pick comes into working contact with the mine face only during part of a revolution. The picks, therefore, need not be supplied with water during their nonworking phases. For this purpose it is known, e.g. as noted in the aforementioned U.S. Pat. No. 3,747,982, to control the water supply by means of a distributor cutting off the flow to a segment remote from the mine face, yet this will not prevent the unneeded emission of water from an opposite segment when the picks thereof are not in contact with the mine face. From European Pat. specification 10,534 it is known to control the water supply to respective nozzles by valves that are opened by adjoining picks when the latter are repressed in their holders against countervailing spring forces.